Copper catalysts are often employed for reactions involving hydrogen, for example simple hydrogenation reactions, e.g. the hydrogenation of aldehydes to alcohols, for methanol synthesis (where carbon oxides are reacted with hydrogen), methanol decomposition (where methanol, often in admixture with steam, is decomposed to form hydrogen and carbon oxides) and the shift reaction (where carbon monoxide is reacted with steam to produce hydrogen and carbon dioxide) and the reverse shift reaction. Often, in order to obtain the optimum activity and stability of the catalyst, the catalyst is made with the copper in a highly dispersed form, for example by precipitation of a copper compound in the presence of, or together with, one or more support materials, especially zinc, magnesium, chromium and/or aluminium compounds. Following such precipitation, the composition is heated to convert the copper compounds, and, if necessary also support materials, to the corresponding oxides. Prior to use for the desired reaction, the copper oxide is reduced to metallic copper. Particularly suitable catalysts for the above reactions are copper/zinc oxide/alumina and copper/zinc oxide/chromia compositions. In some cases part of the zinc may be replaced by magnesium and/or part of the alumina or chromia may be replaced by ceria or a rare earth such as lanthana.
The copper catalysts are readily de-activated by the presence of halide compounds and in particular chloride compounds, such as hydrogen chloride, in the process gas undergoing the reaction. Traces of such chloride compounds may arise from contaminants in the materials, for example hydrocarbon feedstock, steam, or air employed to make the process gas. Such chloride compounds react with the active copper, forming copper chloride. Since copper chloride is relatively low melting, at the temperatures at which the catalysts are commonly employed, e.g. 150–300° C., the copper is mobilised and tends to aggregate resulting in a loss of dispersion of the copper and consequent loss of activity of the catalyst. Also where zinc and/or magnesium oxide is a component of the catalyst, likewise the corresponding chlorides may be formed, and these likewise are liable to be mobilised resulting in loss of the stabilising effect of the zinc or magnesium oxides, again with the consequent loss of dispersion and activity of the copper.
It has been proposed in PCT application WO 01/17674 to employ a guard bed upstream of the copper catalyst wherein the guard bed is a particulate composition containing a lead compound and a support therefor. That application discloses that the guard bed particles may be made by impregnating particles of the support with a solution of a suitable lead salt, for example lead nitrate, by precipitating an appropriate lead compound in the presence of particles of the support material, or by co-precipitating a lead compound and the support, or a precursor to the support. The preferred lead compound was lead nitrate. However, there is a risk when using lead nitrate that, in the event of a plant upset, water may condense on the guard bed and leach the lead nitrate from the support and wash it on to the downstream copper catalyst. Lead compounds tend to poison copper catalysts and so there is the risk that the activity of the copper catalysts may be diminished. For this reason it may be preferable to use a lead compound that is not soluble in water.
In the aforementioned PCT application WO 01/17674 a commercial lead oxide supported on alumina and containing 20.4% by weight of lead was found to be a poor guard bed material compared to other lead compounds tested, providing activity only marginally better than alumina granules.